An antenna is an electrical device which converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an electric current oscillating at radio frequency to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves. In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, applied to a receiver to be amplified. Antennas are essential components of all equipment that use radio. They are used in systems such as radio broadcasting, broadcast television, two-way radio, communications receivers, radar, cell phones, and satellite communications, as well as other devices such as wireless microphones, Bluetooth-enabled devices, and wireless computer networks. Antennas are critical components for military operations.
Modern electronic microsystems can now be made at such low cost that they are increasingly pervasive throughout the battlefield and large numbers can be widely proliferated and used for applications such as distributed remote sensing and communications. However, it is nearly impossible to track and recover every device, resulting in unintended accumulation in the environment as well as subsequent unauthorized use and compromise of technological advantage.
Transient electronics promise a number of revolutionary and meaningful military capabilities of interest including sensors for conventional indoor/outdoor environments (e.g., buildings, transportation, and material), environmental monitoring over large areas, and simplified diagnosis, treatment, and health monitoring in the field. Large-area distributed networks of sensors that can disintegrate into powders independent the natural environment may provide critical data for a specified duration, but no longer. Alternatively, devices that resorb into the body (bioresorbable) could be promising transient electronic implants to aid in continuous health monitoring and treatment in the field. Potential applications and enhanced capabilities are plentiful. The addition of transience as a design variable introduces and may enable new operational strategies.
To date, the challenge of creating fully physically transient electronics including antennas has been addressed primarily through approaches in which the fundamental elements of electronic circuits (actives, passives, etc.) are created from fully physically transient materials and integrated onto transient substrates.
A known approach provides water-soluble devices and substrates (see Hwang et al., Science 337, 1640-1644, 2012). In this approach, transience is achieved through a selection of materials (ultra-thin silicon, magnesium, silk) that trades improved water solubility for degraded electrical performance. While there are scenarios in which such trades are permissible—as in certain in vivo biomedical applications—most military electronics requirements are of a level of performance and complexity out of the reach of even the most sophisticated of transient circuits.
The shortcomings of existing transient electronics solutions are particularly problematic when considered in the context of RF electronics and general-purpose digital logic, for which high operating frequencies and on-off ratios are required. Furthermore, water-soluble materials are at odds with commonly used wafer-processing techniques. This fact, when considered with CMOS foundries' resistance to exotic materials, makes it unlikely that such approaches to transient electronics will be adopted by high-volume manufacturers.
Any solubility-based approach requires either an on-board solvent reservoir, which is easily defeated by removal or deactivation of the release valve; an external supply of the solvent, which cannot be guaranteed in all deployment scenarios; or an on-board means of synthesizing the solvent, which adds complexity to the system design. In any case, these requirements likely restrict the domain of transient electronics technology to low-volume manufacturing and niche applications.
Conventional methods to protect and safeguard military or other intelligence assets involve large-scale combustion or energetic reactions that provide brute-force destruction. While effective in providing a triggered transience, their lack of control on particle size and their potential for detrimental effects on innocent bystanders, handlers, and the environment have severely limited their use.
Most anti-reverse-engineering schemes revolve around protecting a device's software/data and hardware. These schemes fall into two major categories: proactive and reactive. An example of a proactive software scheme is encryption, where a known algorithm and key are used to encode/decode data. An example of a reactive software scheme is the zeroization of data, which repeatedly erases content such that it is physically unrecoverable.
Hardware-based schemes may also be categorized as proactive and reactive. One proactive hardware scheme employs encapsulation or coatings that attempt to prevent probing of the device or silicon die analysis. A reactive hardware protection scheme is overvoltage, which is employed during tamper detection. Overvoltage produces a voltage spike large enough to irreversibly damage the internal circuitry, rendering it nonfunctional. Another less sophisticated, but often used, hardware protection scheme employs explosives or acids to physically destroy the hardware. While reasonably effective, all of the aforementioned protection methods, at best, leave traces behind that could be used to determine vital information and, at worst, endanger the lives of warfighters and threaten the environments in which they are deployed.
Reactive materials for wafer-scale energetics are capable of delivering localized heat, pressure, or shock, which provide a unique capability for protecting sensor and electronic platforms. These materials can generally be classified into four major categories: nanothermites, intermetallics, inkjet-printable secondary, and nanoporous silicon. Nanothermites such as aluminum/copper oxide generate significant heat and pressure. Reaction temperatures can reach upwards of 3000 K while achieving an output energy of 4 kJ/g. Intermetallics like nickel/aluminum have the benefit of generating heat as a byproduct of their exothermic reaction, reaching temperatures of 1600 K. Inkjet deposition of secondary explosives delivers enormous shock waves that can be rapidly transferred to targeted devices. Nanoporous energetic silicon consisting of oxidizer-infused porous silicon can deliver heat, pressure, and shock, making it one of the most versatile reactive materials available.
Among the reactive materials described, nanoporous silicon can be most readily integrated on-chip with MEMS and electronic platforms because the material is fabricated directly from silicon. This monolithic fabrication is done by etching the silicon and converting the device wafer into a silicon fuel that can be infused with an oxidizer. The exothermic reaction can be tailored to achieve a controlled release of energy. While intermetallics can be fabricated using MEMS microfabrication techniques, they can be sensitive and require in-line mixers with ultrasonics and peristaltic pumps. Nanothermites do not readily lend themselves to direct on-chip integration because they are slurry-based materials. While inkjet secondary materials can provide significant, targeted damage, they lack the controlled energy delivery of nanoporous silicon, which can be tuned to deliver precise amounts of targeted energy while residing on-chip.
What are desired are antenna structures and other microsystem components capable of physically disappearing in a controlled, triggerable manner. Transient antennas should maintain designs and performance similar to state-of-art permanent antennas, but with controllable persistence that can be programmed, adjusted in real-time, triggered, and/or be sensitive to the deployment environment.
While there are a wide range of candidate approaches to achieving physical transience (explosives, caustic-chemical-based dissolution, burning, etc.), what are desired are approaches that are fully compatible with existing integrated circuits processing, preserving antenna performance while using foundry-compatible, front-end and post-process techniques.